Gui
From GMod Wiki
GUI Stands for Graphical User Interface. If you're looking for Valve Graphical User Interface (VGUI) try the VGUI library or VGUI on the VDC wiki.
The GUI Library in Garry's Mod allows you to enable mouse control, thereby enabling you to create custom interfaces or mouse controls seen in some RTS/RPG games.
Library Functions
gui.InternalCursorMoved
gui.InternalKeyCodePressed
gui.InternalKeyCodeReleased
gui.InternalKeyCodeTyped
gui.InternalMouseDoublePressed
gui.InternalMousePressed
gui.InternalMouseReleased
gui.InternalMouseWheeled
gui.EnableScreenClicker
gui.MousePos
gui.MouseX
gui.MouseY
gui.OpenURL
gui.ScreenToVector
gui.SetMousePos